pam: cannot remove all arguments of an entry

When setting arguments to `[]` for an entry, the provider doesn't remove them.

This requires to fix #100 first.

Here's a test that fails (in the `with full file` section):

      it "should remove all arguments" do
        apply!(Puppet::Type.type(:pam).new(
          :title       => "Remove type= from pwquality check",
          :service     => "system-auth",
          :type        => "password",
          :control     => "requisite",
          :module      => "pam_pwquality.so",
          :arguments   => [],
          :target      => target,
          :provider    => "augeas",
          :ensure      => "present"
        ))

        aug_open(target, "Pam.lns") do |aug|
          aug.match('./*[type="password" and module="pam_pwquality.so"]/argument').size.should == 0
        end
      end